Ch. 7.2 Volcanic Eruptions

Two general types of lava. Mafic lava—dark when hardened; rich in iron and magnesium; usually thin and flowing; commonly forms ocean floor. Felsic lava—rich in silica…less iron and magnesium; forms continental crust; lighter colored when hardened.

Kinds of Eruptions Lava flow eruptions—usually quiet; produce red-hot rivers of flowing lava. Thin mafic lava. Gases escape easily. Explosive eruptions—thicker felsic lava with lots of trapped gases, produce ash, and various sizes of molten and solid particles shooting into the air.

Volcanic Rock Fragments Pyroclastic material results from exploded felsic lava. The type of pyroclastic material depends on particle size. Volcanic ash—particles less than 2mm in diameter. Volcanic dust—particles less than .25mm. Lapilli—larger particles less than 64mm.

Volcanic bombs—large red-hot lava blobs thrown out of the volcano. Volcanic blocks—solid rock blasted away from the volcano. May be as big as houses.

Volcanic Features Volcanic cone—the buildup of lava and pyroclastic materials around the vent. Three main types…

Shield Volcanoes Broad at the base with gently sloping sides. Result from eruptions of mafic lava.

Cinder Cones Only a few hundred meters high at most; very steep sides. Result from explosive eruptions of solid fragments.

Stratovolcanoes (Composite Volcanoes) Formed when a volcano experiences both quiet lava flows and explosive eruptions. Alternating layers of different materials build up into very high volcanic mountains.

Crater—the funnel-shaped pit at the top of a volcanic vent. Caldera—large basin-shaped depression formed when a volcanic cone collapses because the magma chamber below empties (Ex: Yellowstone Park).

Predicting Eruptions Small earthquakes often increase in frequency and strength just before eruptions. There may be slight bulging of the volcano’s surface prior to an eruption. There may be changes in gases coming out of the volcano. Past history is important.